1. Field of the Invention
This invention relates to metal bore seals for sealing connections between oil and gas well tubular members and other equipment in sub-sea drilling and production operations. More specifically, the invention relates to a corrosion-resistant bimetallic compression ring designed for both well pressure containment and deep water pressure resistance.
2. Description of the Prior Art
Many oil and gas fields are situated offshore at various ocean depths. In the oil and gas exploration and production industries, the underwater connection between the wellhead and the rig or platform is achieved by sequentially joining together items of drilling and production equipment, as well as long segments of metal and composite pipes, or riser joints, to form a pressure tight system. Metal bore seals (or gaskets) are commonly used to create and maintain the requisite pressure tight seal at the joints. These bore seals therefore provide metal-to-metal pressure tight seals for up to 15,000 pounds force per square inch (lbf/in2) between the wellhead, connectors, and other equipment used to attach drilling and production equipment to the main well bore, or annulus.
The bore seal in these applications is a compression ring disposed and compressed between the opposing ends of the tubular members to be joined. Prior art bore seal designs conform to several profile types, such as AX, CX, DX, LX, NX, and VX, and generally have an internal diameter matching the bores of tubular members and equipment to be joined and sealed. Typically, the ring is about four inches long (measured in its axial direction) and is machined externally to provide two conical faces tapering towards each end at about a twenty-three degree angle to the axis. The ends of the connecting components are machined to open outwardly in order to provide tapering conical seats for the ring at the same angle but with a slightly smaller diameter. “Energization” or seating of the ring requires bringing the opposing tubular members and the bore seal together with considerable force, and is achieved by one of three methods: (1) hydraulic actuators in one of the opposing components, (2) stud bolts and flanges, or (3) hubs and clamps.
When the necessary force is applied to bring the tubular members and bore seal into assembly, the disparity in diameter between the tubular members and the bore seal produces compressive stress in the seal. “Preloading” is the term for this procedure. The magnitude of the diametral disparity or interference and the yield strength of the seal material govern the contact force between the conical faces. Generally, the preload stress in the bore seal exceeds yield and causes some permanent compressive strain. After the bore seal is properly seated, pressure in the well annulus imparts further energization to the ring thereby adding to that created by the compressive force during preloading.
Bore seals are critical components and their materials need to meet several, sometimes conflicting, requirements. One prior art requirement is that the bore seal should achieve its yield stress during preloading but not have a hardness that causes damage to the seating areas in the connecting tubular members. Thus, a general preference in the manufacture of bore seals is to use low yield strength materials that are no harder than the seat surfaces of the connecting tubular members to minimize the risk of galling during preloading of the connection. These bore seal material requirements are met by specifying Brinell hardness numbers in the range between 110 and 170, which produce yield strengths of 35,000 to 40,000 lbf/in2 in the low carbon steels and series 300 stainless steels employed for bore seal service at bore pressures up to 15,000 lbf/in2 and maximum temperatures of 250° F. The materials described are suitable for sour well service although the low carbon steel is inferior to the stainless steels with regard to overall corrosion resistance. The low carbon steels have limited resistance to extremely corrosive well conditions. Conversely, stainless steels, such as type 316L in particular, have a much wider application.
The design of bore seals has, in the past, been primarily concerned with internal pressure, a mode of operation assisted by bore pressure energization. Bore seals made of low carbon steel or the 300 series stainless steels, being at or close to yield at preload, may suffer an increase of permanent compressive strain during service due to bore diameter changes from rises in temperature and pressure fluctuations. These effects will be greater with the 300 series stainless steels, because their thermal expansion coefficients are about one third greater than those of the low alloy steels used in the manufacture of the tubular members and other equipment. Ideally, the expansion coefficient of the bore seal material should closely match that of the tubular members in between which the bore seal is seated, in order to minimize this effect.
Increases of permanent strain brought about by operating conditions will reduce the contact force created between the seal and seat when the connection was preloaded, although this is unlikely to affect performance if bore pressure and temperature are substantially maintained. However, if the bore pressure and temperature are greatly reduced, the reduction of the diametral interference between seal and seat may result in failure of the connection to retain pressure.
Bore seal materials must be able to resist corrosion by the produced well fluids. In particular, they must resist stress corrosion cracking caused by acid gases, such as H2S and CO2, dissolved in the liquid phase. This requirement, however, limits the hardness of the bore seal and consequently its material strength. Furthermore, this hardness limitation is in direct conflict with the need for adequate yield strength to resist external pressures, as encountered in deep water operations.
The trend towards deep water exploration at depths exceeding 2,000 feet is the result of oil and gas depletion in shallower offshore waters. Normally, the internal pressure within the well bore or annulus is maintained to equal or exceed the external water pressure. However, as a result of unforeseen behavior in the formation being drilled, a sudden loss of bore pressure to a value below the external pressure can occur.
The low carbon and series 300 stainless steel designs, however, cannot function reliably against external pressure because their low strength levels make them prone to incremental permanent strain due to operational variations of pressure and temperature. The strength of low carbon and series 300 stainless steels is also reduced at elevated temperatures. For the stainless steels often employed in these applications, this reduction in strength can be as much as about 20% at 250° F., which confirms the temperature classification limit previously disclosed.
Notwithstanding cost, bore seal material selection requires the consideration of several factors including material strength, hardness, and corrosion resistance. Several prior art patents have attempted to address this difficulty in bore seal material selection. U.S. Pat. No. 3,507,506 issued to Tillman discloses a sealing ring constructed of a metal, such as steel, and having sealing surfaces with inlays. The sealing ring is coated with different types of plastic flow or elastic material such as Teflon®, Hycar®, rubber, plastic, etc. However, coatings of this type are not durable enough to resist abrasion and corrosion for more than a short period of time. Alternatively, the sealing ring may be coated with silver, lead, zinc, cadmium, copper, etc. to provide a redistributable surface and thereby a tighter joint. However, electroplated metals such as silver, lead, zinc, cadmium or copper can be applied in a thickness range of only 12 to 25 microns and are therefore unable to resist corrosion or abrasion for more than a short period of time.
U.S. Pat. No. 4,635,967 issued to Stephenson discloses a bore seal ring constructed of a corrosion resistant alloy, such as stainless steel. The sealing surfaces of the '967 bore seal ring are preferably protected from corrosion and/or oxidation in the elevated temperature environment by an appropriate plating or coating. Stephenson further discloses that a plating, such as a electrodeposited nickel over hard copper, was successfully used. While electrodeposited nickel can be deposited in thicknesses that will withstand corrosion for extended periods, such material is unsuitable for bore seals because of its extreme hardness and difficulty of machining.
U.S. Pat. No. 6,722,426 issued to Sweeney discloses a composite metal sealing ring constructed of a corrosion resistant alloy, preferably a high strength stainless steel having a yield strength at least from 35,000 to 40,000 lbf/in2. A molybdenum sulfide coating is then applied onto the sealing surfaces to provide lubrication and prevent galling from occurring. While lubricating compounds can be successfully applied, e.g. Teflon® in multiple applications to a thickness of 50 to 60 microns and molybdenum sulfide to 8 to 12 microns, they are also unable to resist abrasion and corrosion for more than a short period of time.
U.S. Pat. No. 7,025,360 and UK Patent Application No. GB2407850A issued to Walker et al. disclose a bimetallic bore seal technology which involves welding a partial stainless steel or corrosion resistant alloy inlay to the first and second outer sealing faces of the bore seal to protect against corrosion at those locations. Due to the partial stainless steel or corrosion resistant inlay, steel with a similar co-efficient of expansion to that of the tubular members can be used to make the remaining bore seal structure. Welding a corrosion resistant inlay onto the bore seal surface solves some of the previously discussed problems; however, it is not an ideal solution. As a partial deposit on the sealing surfaces, the interface between the steel body and welded corrosion resistant alloy inlay may be subject to crevice corrosion. After the corrosion resistant or lubricant coating which is applied over the partially welded gasket has eroded, accelerated corrosion could occur due to bimetallic contact with aqueous electrolyte. Furthermore, in some cases, where the inlay has a high thermal expansion the bore seal may act as a bimetallic strip causing its tips to flex away from the seat areas.
Partial or complete weld coverings have additional potential disadvantages when used to protect and/or strengthen the bore seal core materials. The process of weld deposition generates extreme heat that may affect the mechanical properties of the core bore seal material. The extreme heat may also cause changes to the physical shape of the core bore seal material which can result in post-weld difficulties in machining the bore seal to an accurate shape profile. As previously indicated, crevice corrosion between the weld lines of the bore seal core material and the corrosion resistant inlay may also become apparent over time, especially if there are any imperfections in the welds.
With the current trend towards deep water exploration and drilling, the problem of external pressure comes into greater prominence. Coupled with this, higher temperatures and more aggressive corrosive conditions are encountered as drilled depths increase. All of these factors combine to render obsolete the emphasis on internal pressure containment. The requirement for pressure sealing against deep water external pressures thus imposes two essential features which are lacking in the existing generation of bore seal designs: 1) greater yield strengths which exceed the maximum operating stresses and which enable the seals to work within the elastic limit, and 2) the provision of corrosion resistant alloys suitable for the high-temperature, aggressively-corrosive conditions likely to be experienced in deeper wells, alloys which also facilitate the use of a harder, high-strength, low-alloy steel core.
Greater emphasis on the selection of corrosion resistant materials, such as nickel-based alloys, for bore seal construction, is thus required. Two nickel alloys, alloy 625 and alloy 825 (Inconel®), are specified for use as bore seal materials of construction in severely corrosive conditions with service temperatures up to 400° F. By controlling their heat treatment, these two alloys are used at yield strengths in the range of 40,000 to 70,000 lbf/in2. Moreover, the hardness of each of these alloys is only marginally higher than the hardness of series 300 stainless steels. The nickel alloys 625 and 825 have expansion coefficients which are 10% and 20% higher, respectively, than a low alloy steel. However, a major drawback to the prevalent use of these nickel alloys is their expense. Constructing a metal bore seal completely from a corrosion resistant nickel alloy would solve some of the problems previously discussed, however, such construction would be cost prohibitive in many applications.
The foregoing illustrates a few of the shortcomings of the current sub-sea bore seals in relation to the evolving requirements for deep water drilling and production operations. Thus, a metal bore seal, and method of manufacturing same, which avoids the aforementioned limitations of the prior art, incorporates nickel-based corrosion resistant alloys, and remains cost effective is highly desirable.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a bimetal bore seal, and method of manufacturing same, for sealing connections between oil and gas well tubular members that has the material properties capable of resisting aggressively corrosive conditions and higher temperatures encountered in deeper oil and gas wells, and also resisting external pressure in deep seawater.
Another object is to provide a bimetal bore seal of sufficient yield strength to provide adequate elasticity following a decline from maximum operating conditions to nil bore pressure.
Another object is to provide a bimetal bore seal which has a good strength retention in order to enable the bore seal to operate within its elastic limit and thus maximize its service temperature range.
Another object is to provide a bimetal bore seal with an overall yield strength to withstand external seawater pressure differentials up to 7,500 psi.
Another object is to provide a bimetal bore seal constructed to resist stress corrosion cracking caused by acid gases dissolved in the liquid phase.
Another object is to provide a bimetal bore seal with a metal core constructed from any high strength ferrous material in the range of yield strength up to 180,000 lbf/in2 at room temperature and coated in its entirety with a stainless steel or nickel-based corrosion resistant alloy, which is applied by a thermal spray process.
Another object is to provide a bimetal bore seal with a high strength, high hardness metal core and a corrosion resistant metal coating, which is applied by a thermal spray process to a hardness that is lower than the hardness limit specified by NACE International.
Another object is to provide a bimetal bore seal having commercially available profiles, such as AX, CX, DX, LX, NX, or VX.
Other objects, features, and advantages of the invention will be apparent to one skilled in the art from the following specification and drawings.